In Chromium and Firefox on a height=1080 width=1920 display with 150% zoom, https://docs.freebsd.org/en/books/fdp-primer/rosetta/#docbook-vs-asciidoc only display the horizontal scroll bar when I'm at the bottom of that section. Thus, to see the AsciiDoc equivalent for a Docbook feature, I need to use the arrow keys or page down to the bottom, scroll to the right using the now-visible scrollbar, and page back up. This is more an annoyance than a genuine problem for me since keyboard scrolling works on my laptop, but it should still be fixed in case it also affect tablet and smartphone users.
Side note: the four corresponding pages at <https://download.freebsd.org/doc/en/books/fdp-primer/fdp-primer_en.pdf#%5B%7B%22num%22%3A203%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C0%2C765.17%2Cnull%5D> would be better landscape than portrait. I assume that an automated mixture of portrait and (non-rotated) landscape is not possible, or not easy, with the current infrastructure for documentation.